The teachings of the present invention are applied to silicon steel having a cube-on-edge orientation, designated (110) [001] by Miller's Indices. Such silicon steels are generally referred to as grain oriented silicon steels. Grain oriented silicon steels are divided into two basic categories: regular grain oriented silicon steel and high permeability grain oriented silicon steel. Regular grain oriented silicon steel utilizes manganese and sulfur (and/or selenium) as the principle grain growth inhibitor and generally has a permeability at 796 A/m of less than 1870. High permeability silicon steel relies on aluminum nitrides, boron nitrides or other species known in the art made in addition to or in place of manganese sulphides and/or selenides as grain growth inhibitors and has a permeability greater than 1870. The teachings of the present invention are applicable to regular grain oriented silicon steel.
Conventional processing of regular grain oriented silicon steel comprises the steps of preparing a melt of silicon steel in conventional facilities, refining and casting the silicon steel in the form of ingots or strand cast slabs. The cast silicon steel preferably contains in weight percent less than about 0.1% carbon, about 0.025% to about 0.25% manganese, about 0.01% to 0.035% sulfur and/or selenium, about 2.5% to about 4.0% silicon with an aim silicon content of about 3.15%, less than about 50 ppm nitrogen and less than about 100 ppm total aluminum, the balance being essentially iron. Additions of boron and/or copper can be made, if desired.
If cast into ingots, the steel is hot rolled into slabs or directly rolled from ingots to strip. If continuous cast, the slabs may be pre-rolled in accordance with U.S. Pat. No 4,718,951. If developed commercially, strip casting would also benefit from the process of the present invention. The slabs are hot rolled at 2550.degree. F. (1400.degree. C.) to hot band thickness and are subjected to a hot band anneal of about 1850.degree. F. (1010.degree. C.) with a soak of about 30 seconds. The hot band is air cooled to ambient temperature. Thereafter, the material is cold rolled to intermediate gauge and subjected to an intermediate anneal at a temperature of about 1740.degree. F. (950.degree. C.) with a 30 second soak and is cooled as by air cooling to ambient temperature. Following the intermediate anneal, silicon steel is cold rolled to final gauge. The silicon steel at final gauge is subjected to a conventional decarburizing anneal which serves to recrystallize the steel, to reduce the carbon content to a non-aging level and to form a fayalite surface oxide. The decarburizing anneal is generally conducted at a temperature of from about 1525.degree. to about 1550.degree. F. (about 830.degree. to about 845.degree. C.) in a wet hydrogen bearing atmosphere for a time sufficient to bring the carbon content down to about 0.003% or lower. Thereafter, the silicon steel is coated with an annealing separator such as magnesia and is box annealed at a temperature of about 2200.degree. F. (1200.degree. C.) for twenty-four hours. This final anneal brings about secondary recrystallization. A forsterite or "mill" glass coating is formed by reaction of the fayalite layer with the separator coating.
Representative processes for producing regular grain oriented (cube-on-edge) silicon steel are taught in U.S. Pat. Nos. 4,202,711; 3,764,406; and 3,843,422.
The present invention is based upon the discovery that in the conventional routing given above, the hot band anneal can be eliminated if the intermediate anneal and cooling practice of the present invention is followed. The intermediate anneal and cooling procedure of the present invention contemplates a very short soak preferably at lower temperatures, together with a temperature controlled, two-stage cooling cycle, as will be fully described hereinafter.
The teachings of the present invention yield a number of advantages over the prior art. At all final gauges within the above stated range, magnetic quality is achieved which is at least equal to and often better than that achieved by the conventional routing. The magnetic quality is also more consistent. The teachings of the present invention shorten the annealing cycle by from 20% or more, thereby increasing line capacity. The process of the present invention enables for the first time the manufacture of thin gauge, typically about 9 mils (0.23 mm) to about 7 mils (0.18 mm), regular grain oriented silicon steel having good magnetic characteristics without a hot band anneal following hot rolling to hot band. This enables thin gauge regular grain oriented silicon steel to be manufactured where hot band annealing can not be practiced. The lower temperature of the intermediate anneal of the present invention increases the mechanical strength of the silicon steel during the anneal, which previously was marginal at high annealing temperatures.
European Patent No. 0047129 teaches the use of rapid cooling from 1300.degree. to 400.degree. F. (705.degree. to 205.degree. C.) for the production of high permeability electrical steel. This rapid cooling enables the achievement of smaller secondary grain size in the final product. U.S. Pat. No. 4,517,932 teaches rapid cooling and controlled carbon loss in the intermediate anneal for the production of high permeability electrical steel, including an aging treatment at 200.degree. to 400.degree. F. (95.degree. to 205.degree. C.) for from 10 to 60 seconds to condition the carbide.
These high permeability silicon steel references employ a very low temperature and lengthy intermediate anneal cycle having a 120 second soak at 1600.degree. F. (870.degree. C.) followed by rapid cooling from 1300.degree. F. (705.degree. C.) and an aging treatment to condition the carbide precipitates. It has been found, however, that in the intermediate anneal of the present invention, rapid cooling from above about 1150.degree. F. (620.degree. C.) or higher produces poorer magnetic quality owing to the formation of martensite which increases hardness, degrades mechanical properties for subsequent cold rolling, and contributes to poorer magnetic quality in the final product.
In the above-noted U.S. Pat. No. 4,517,032, a low temperature aging treatment following rapid cooling is employed. This practice, if used for regular grain oriented materials, has been found to produce enlarged secondary grain size and poorer magnetic quality in the final product since it impaires the fine iron carbide precipitates. Lower temperature annealing at about 1640.degree. F. (895.degree. C.) or lower, to avoid the formation of austenite, could be used to provide adequate solution of iron carbide without forming a second phase which must be conditioned out of the microstructure. However, this procedure requires much longer annealing times to effect carbide solution. Such a procedure would permit direct rapid cooling from soak temperature without the two-stage cooling cycle of the present invention.
U.S. Pat. No. 4,478,653 teaches that a higher intermediate anneal temperature can be used to produce 9 mil (0.23 mm) regular grain oriented silicon steel without hot band annealing. It has been found, however, that 9 mil (0.23 mm) regular grain oriented silicon steel made in accordance with this patent has more variable magnetic quality than when a routing utilizing a hot band anneal is used. It has further been found that the no hot band anneal-high temperature intermediate anneal practice taught in this reference provides generally poor magnetic quality at thinner gauges of 9 mils (0.23 mm) or less, when compared to the above noted practice employing a hot band anneal. Finally, the very high temperature of the intermediate anneal of U.S. Pat. No. 4,478,653 results in low mechanical strength of the silicon steel, making processing more difficult.